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Letters to the Editor |
1 CNR Institute of Clinical, Physiology of Pisa, Pisa, Italy
2 Clinical Chemistry Laboratory, San Bortolo Hospital, Vicenza, Italy
3 Clinical Immunology Unit, Department of Internal Medicine, University of Pisa, Pisa, Italy
4 Scuola Superiore Sant’Anna, Pisa, Italy
aAddress correspondence to this author at: Laboratory of Cardiovascular Endocrinology and Cell Biology, C.N.R. Institute of Clinical Physiology, Via Trieste 41, 56126 Pisa, Italy. Fax 39-0585-493601; e-mail clerico{at}ifc.cnr.it.
To the Editor:
In light of recommendations on the quality (1) and clinical use (2) of troponin assays, we evaluated the analytical performance of the ADVIA Centaur and ADVIA CP® platforms (TnI-Ultra, Siemens Medical Solutions Diagnostics SrL) for measurement of cardiac troponin I (cTnI). The chemiluminescent TnI-Ultra method uses 2 monoclonal capture antibodies directed to epitopes at amino acids 41–49 and 87–91 and a tracer polyclonal goat antibody labeled with acridinium ester, directed against amino acids 27–40 (1)(3)(4). Two clinical laboratories participated in the study: the CNR Institute of Physiology in Pisa and the San Bortolo Hospital in Vicenza.
The limit of detection (limit of the blank) for the TnI-Ultra method was calculated as the concentration corresponding to a signal of 3 SD above the mean of 60 replicates (obtained in 4 different runs and pooled together) for the calibrator in which cTnI was absent; a mean cTnI concentration of 0.006 µg/L was found. The total imprecision (CV%) of the TnI-Ultra method, assessed according to the NCCLS EP5-A protocol over 20 consecutive working days, was 11.6%, 5.6%, and 4.4% for 3 plasma samples with cTnI concentrations of 0.05, 0.25, and 2.68 µg/L, respectively. From plots of CV vs log-transformed values of cTnI concentration in the range 0.006–0.20 µg/L, the cTnI concentrations that corresponded to 10% CV were 0.064 µg/L for ADVIA Centaur CP® and 0.07 µg/L for ADVIA Centaur.
Blood samples, collected in polypropylene tubes with lithium heparin, were used in the study, according to the routine protocol adopted by both clinical laboratories. The 2 laboratories enrolled a white population including 418 apparently healthy adult individuals (204 men and 214 women) with a mean (SD) age of 50.7 (16.6) years, range 16–89 years; the mean (SD) age in women was 52.6 (17.5) years and in men 48.7 (15.5) years. The presence of cardiac or other acute or chronic diseases was excluded by clinical examination and laboratory tests. Informed concent was obtained by all individuals and patients before testing, and the study protocol was approved by the local ethics committee. The measured cTnI values approximated a log-normal distribution with a calculated 99th percentile of 0.087 µg/L; therefore, the ratio of 10% CV concentration to 99th percentile limit for the TnI-Ultra method was 0.067:0.087 = 0.77 (1). In 82 samples, including 81 females and only 1 male, we found values <0.004 µg/L (i.e., undetectable cTnI concentration), and so an arbitrary concentration of 0.001 µg/L was attributed to these samples. A highly significant correlation was found between cTnI values and age (R = 0.268, P <0.0001 by Spearman rank correlation coefficient test). Moreover, a significant difference was found between the cTnI values found in men and women, respectively [mean (SD) 0.015 (0.018) µg/L, median 0.012 µg/L, range 0–0.196 µg/L, n = 204 for men; 0.009 (0.014) µg/L, 0.008 µg/L, 0–0.130 µg/L, n = 214 for women; P <0.0001 by Mann–Whitney U-test]. We found that both sex (as a dummy independent variable with F = 1 and M = 2) and age (as a continuous independent variable) independently contributed to the regression with cTnI (as a dependent variable after log transformation of original values) by using a stepwise multiple regression analysis (log cTnI = –3.164 + 0.456 sex + 0.007 age; P <0.0001, F-value = 71.962, R = 0.508, n = 416).
A close linear relationship was found between cTnI values measured by ADVIA TnI-Ultra with the Centaur CP® platform and the Access AccuTnITM method on the UniCell® DxI 800 platform (Beckman Coulter) in 318 plasma samples of 155 apparently healthy individuals and 163 cardiac patients (ADVIA= 0.016 + 1.272 Access; R = 0.936). The TnI-Ultra method showed higher cTnI values than the Access AccuTnI method (on average by 22.0%; P <0.0001 by Wilcoxon signed-rank test) and based on the 99th percentile values for each assay, 9 discordances were found between assays for values within the reference interval vs increased values.
The ADVIA TnI-Ultra method showed no interference from dilutions with plasma samples that contained high concentration of triglycerides (6.6 g/L, final dilution 1:128; y = –0.044 + 0.14x, n = 8, R = 0.99) or hemoglobin (1.47 g/L, final dilution 1:4996; y = 0.04 + 0.060x, n = 13, R = 0.99). No apparent positive interference was seen in 58 patients with symptomatic rheumatoid arthritis [10 men and 48 women, mean (SD) age 60.8 (10.2) years] with a mean concentration of rheumatoid factor of 189.6 kIU/L (range 40–1280 kIU/L), because the mean (SD) cTnI concentration was not increased 0.017 (0.023) µg/L.
The present study indicates that the ADVIA TnI-Ultra method meets the quality specifications recommended by NACB and IFCC Committee for the Standardization of Cardiac Damage (5).
Acknowledgments
Grant/funding support: None declared.
Financial disclosures: None declared.
References
The following articles in journals at HighWire Press have cited this article:
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  D. v. d. Kerkhof, B. Peters, and V. Scharnhorst Performance of the Advia Centaur second-generation troponin assay TnI-Ultra compared with the first-generation cTnI assay Ann Clin Biochem, May 1, 2008; 45(3): 316 - 317. [Abstract] [Full Text] [PDF]
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